This invention relates to a photosensitive resin composition for forming conductor patterns so as to realize a build-up method wherein a photosensitive resin composition coated on an insulating substrate is activated by exposing to light so as to have a catalyst selectively on the insulating substrate, followed by plating thereon to form conductor patterns, a multilayer printed circuit board using said resin composition, and a process for producing said multilayer printed circuit board.
With a remarkable development of the degree of integration in semiconductor integrated circuits and a development of surface mounting technique for directly mounting small chip parts, there has been demanded to form circuit conductors with higher density on printed circuit boards.
Higher density of printed circuit boards was attained by improving conductor density in a substrate plane heretofore. But in order to apply the recent developments of electronic parts and parts mounting technique mentioned above to such a demand, there is a limit in improving the density in a plane, and it becomes necessary to form a multilayer structure of circuit conductors by piling circuit conductors, that is, to form a so-called multilayer printed circuit board.
According to a previous technique, a multilayer printed circuit board was produced by etching a copper-clad laminate which has a copper foil on one side of an insulating substrate to form a circuit conductor, piling a number of such conductor-formed laminates with heating and pressing, drilling holes for through-holes and via holes, plating copper into these holes to connect individual conductors. But such a previous technique has the following disadvantages. (i) There takes place misregistration of individual conductor layers at the time of heating and pressing, resulting in mis-connection of individual conductor layers after copper plating. Thus the production cost increases with the number of conductor layers. For example, if the production cost for a two-layered printed circuit board is assumed 1, the production cost becomes 2 for four-layered board, 4 for six-layered board, 10 for eight-layered board. (ii) It is difficult to form very small through-holes and via holes by drilling, therefore the diameter of 0.3 mm is the limits. In order to improve the density of circuit conductors, it is desirable to make the diameter of holes smaller, said holes only having a function for connection. Recently, the diameter of holes is desired to make 0.2 to 0.1 mm for such a purpose. This is impossible according to the previous technique.
Taking the place of the previous technique, it has been proposed a so-called build-up method wherein insulating layers and circuit layers are piled alterately successively.
The build-up method has an advantage of forming very fine through-holes and via holes, since these through-holes and via holes are formed by a light exposure-developing method, a dry etching method using sputtering, etc. Further, since circuit conductor layers are successively laminated, misregistration of individual layers hardly takes place, resulting in a higher yield in production. But this method is still insufficient technically, so that this method is not used in practical production.
On the other hand, as conductor pattern forming methods for realizing the build-up method, there have been proposed the following methods.
Japanese Patent Examined Publication No. 50-40221 discloses a process for forming a printed circuit comprising mixing a metal powder which becomes a nucleus for electroless copper plating with a photosensitive resin, exposing the photosensitive resin to light in conductor pattern-wise to cure the resin, removing the photosensitive resin in unexposed portions, conducting electroless copper plating on the surface of conductor pattern-wisely exposed photosensitive resin to form a conductor pattern. But this method has the following disadvantages.
(i) In order to form conductor patterns by electroless copper plating, it is necessary to mix the metal powder, which becomes plating nucleus with the photosensitive resin, in an amount of equivalent weight or more to photosensitive resin. But when a large amount of metal powder is added to the photosensitive resin, the metal powder intercepts the light (generally ultraviolet light) for exposure so as to prevent the light from reaching deep portions of the resin layer. Thus, photocuring becomes inadequate in the deep portions of the resin layer. When the photosensitive resin in unexposed portions is removed (e.g. by spraying a solvent, developing and removing), the remaining resin layer (exposed portion) is damaged, or swollen to fail to obtain very fine conductor patterns.
(ii) When the metal powder is mixed in an amount of less than equivalent weight to the photosensitive resin, the number of plating nucleus becomes inadequate to electroless copper plating, resulting in spot-like deposition of copper. Therefore, good conductor patterns cannot be obtained.
Japanese Patent Examined Publication No. 56-36598 discloses a process for forming an electroconductive metal layer on an insulating substrate comprising mixing a thermosetting resin with a photosensitive semiconductor oxide particles such as TiO.sub.2, ZnO, etc., coating the resulting thermosetting resin composition on a whole surface of a support (insulating substrate), semi-curing the thermosetting resin with heating, exposing the thermosetting resin conductor pattern-wise to light to deposit a noble metal, which becomes a plating nucleus, using electrons produced on the surface of TiO.sub.2, ZnO or the like by light exposure, forming a conductor pattern by electroless copper plating, and curing the thermosetting resin completely to generate adhesive strength between the conductor pattern and the thermosetting resin. But this method has the following disadvantages.
(i) Since the thermosetting resin per se has no pattern forming ability, it is necessary to form a pattern structure by screen printing. According to the screen printing, it is only possible to form a pattern with line width of about 0.5 mm at least, and it is impossible to form, for example, very fine via holes having a very small diameter of 0.1 mm.
(ii) When the thermosetting resin is cured with heating, flowing of the resin is generated to form a tough and fine skin layer on the surface thereof. An epoxy resin is a typical example of the thermosetting resin. In order to expose surfaces of the photosensitive semiconductor oxide particles over the resin, it is necessary to remove the skin layer. But in practice, it is impossible to remove the skin layer. Thus, an etching method is employed during semi-cured state. But since the thermosetting reaction is so rapid that a constant semi-cured state cannot be obtained, resulting in failing to form conductor patterns stably.